Methane is associated in varying amounts with most coal deposits. It may be formed thermogenically during burial and thermal maturation of the coal or it may be produced biogenically by the action of microbes. Bacteria are considered to be the primary degraders of compounds in coal, producing a range of intermediates which are successively degraded to methane precursors, such as hydrogen gas, carbon dioxide, acetate and various other compounds (e.g. dimethyl sulfide, formate, methanol and methylamines). These precursors are then converted to methane via methanogenic archaea. This methanogenic process may occur via a number of mechanisms including CO2 reduction, acetoclastic (from acetate) processes and methylotrophic processes.
The coal seam environment in which biogenic methane is produced is anoxic and reducing. Due to macronutrient limitation biogenic methane production is slow and occurs over long time-scales.
Production from a typical coal seam methane (CSM) well may occur for 5-7 years, after which time, the rate of production generally becomes uneconomic and the well may be abandoned.
It may be possible to prolong the production life of the well by introducing methanogenic microbial populations. For example, U.S. Publication No. 2004/0033557 describes introducing a consortium of selected anaerobic microorganisms into a subsurface formation for in situ conversion of organic compounds into methane and other compounds.
Furthermore, it may also be possible to relatively rapidly replenish the methane within a buried coal seam by stimulation/invigoration of the microbes that reside in the coal and/or associated water. It is known that this can be achieved by addition of nutrients to the system. For example, U.S. Pat. No. 7,832,475, the relevant contents of which are incorporated herein by reference, describes a method for enhancement of biogenic methane production that involves introducing an indiscriminate microbial population stimulant, such as corn syrup, emulsified oil, and/or milk, to blanket boost microbial populations in a hydrocarbon-bearing formation. The method further involves subsequent manipulation of the microbial populations by selectively starving one or more microbial populations to selectively sustain at least one of the boosted microbial populations.
In particular, U.S. Pat. No. 6,543,535, the relevant contents of which are incorporated herein by reference, provides a process to enhance methane recovery through a process of methodically analysing the microbial consortia and its subterranean environment to determine what changes are required in the ecological environment to promote microbial generation of methane. While the underlying principles have been beneficial in enhancing methane recovery in a significant number of hydrocarbon bearing subterranean formations, sustained in situ microbial activity is not always achieved.
Furthermore, U.S. Pat. No. 6,543,535 discloses a process for stimulating the activity of microbial consortia in a hydrocarbon-bearing, subterranean formation to convert hydrocarbons present to methane gas. In summary, the process includes the steps of: (i) analysing the formation; (ii) detecting and characterising the microbial consortia; (iii) utilising the previously acquired information to determine ecological conditions that promote in-situ microbial methane production; and modifying the formation environment accordingly to stimulate microbial conversion of hydrocarbons to methane. Furthermore, this document teaches that the addition of suitable substances to promote the growth of the microorganism include nitrogen and phosphorus. However, U.S. Pat. No. 6,543,535 is primarily directed to optimal methane production from a liquid carbonaceous medium, in particular from oil reservoirs. In contrast to liquid carbonaceous medium, solid carbonaceous medium, such as coal, is more heterogeneous and the reaction kinetics often substantially slower. As a result, the teachings of U.S. Pat. No. 6,543,535 cannot be directly transferred to a solid carbonaceous medium in order to achieve sustainable methane production.
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.
An object of a preferred embodiment of the invention is provide a process of further accelerating and sustaining biochemical conversion of solid hydrocarbons to methane at a rate that is commercially practical or establishing and/or maintaining an in situ environment that supports commercial rates of hydrocarbon conversion and methanogenesis.
Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.